Process of making a subtracted image radiographic record

ABSTRACT

Method and apparatus using an electron radiographic system for making a subtracted image record of an object. The method includes the steps of making the first exposure with a first polarity for the field between the electrodes, reversing the polarity and making a second exposure on the same dielectric receptor, and developing the resultant electrostatic image, typically with a contrast material being introduced into the object between the exposures.

ilnite States ?atent n 1 Proudian 1 1 PROCESS OF MAKING A SUBTRACTED IMAGE RADHOGRAPHIC RECORD [75] Inventor: Andrew P. Proudian, Chatsworth,

Calif.

[73] Assignee: Xonics, lnc., Van Nuys, Calif.

[22] Filed: Feb. 28, 1973 [21] App]. No.: 336,709

[1 1 3,813,546 May 28, 1974 9/1970 Roth ct al. 250/315 10/1970 Tsukatani et al. 250/315 Tinsley [57] ABSTRACT Method and apparatus using an electron radiographic system for making a subtracted image record of an object. The method includes the steps of making the first exposure with a first polarity for the field between the electrodes, reversing the polarity and making a second exposure on the same dielectric receptor, and developing the resultant electrostatic image, typically with a contrast material being introduced into the object between the exposures.

2 Claims, 1 Drawing Figure [51] Int. Cl. G031) 41/16 [581 Field 01 Search 250/315, 327, 315 A [56] References Cited UNITED STATES PATENTS 2,866,903 12/1958 Bcrchtold 250/315 3,057,997 10/1962 Kaprelian.... 3,194,131 7/1965 Robinsonm, 3,484,752 12/1969 Kallmann 250/315 HIGH l/0LT46E./27 SUPPLY PATENTEDIIAY 28 m4 3.813546 HIGH VOLTAGE/Z 7 l .suppu H/ 6 H VOLTHGE su p; Y

PROCESS OF MAKING A SUBTRACTED IMAGE RADIGGRAPHIC RECORD This invention relates to the creation of X-ray images without the use of conventional X-ray film, sometimes referred to as ionography or electron radiography. In particular, it relates to the technique described in the copending application of Muntz, et al, Ser. No. 261,927, filed lune lg 1972, entitled Radiographic Systems with Xerographiclrinting, and assig nedfothe same assignee as the present application. In such a system, an X-ray opaque gas at high pressure (e.g. atmospheres) is used between two electrodes in an imaging chamber to produce a photoelectric current within that chamber as a function of X-rays entering the chamber. The current is collected on a dielectric sheet placed on one or the other of the electrodes, resulting in a latent electrostatic image on the sheet. The latent image is then made visible by xerographic techniques. Collection of the primary photoelectrons created by the X-rays absorbed in the interelectrode gas filled gap and of the secondary electrons created by collisions of the primaries with the gas atoms is achieved by use of an accelerating potential difference, typically of the order of 5,000 volts, applied between the electrodes.

More particularly, the invention is related to the use of electroradiography in the production of subtracted image pictures or records. Subtraction is a technique used in medical radiography to enhance the visibility of certain features, such as blood vessels, in a radiographic record such as an X-ray picture. This enchancement is achieved by making two radiographic exposures, the first with no contrast or radiopaque medium present in the features to be made visible (e.g. blood vessels), the second after administration of a contrast medium. In the conventional system, the first exposure is used to make a so-called base film. A subtraction mask is made from this initial radiograph' which mask is a reverse-tone facsimile of the initial radiograph. After the radiopaque material is introduced into the imaging area, a second radiograph is made. When the reverse tone facsimile of the first radiograph is placed in register over the second radiograph, the density observed over the image will be essentially uniform except in those areas where the radiographs differ, provided the reverse tone facsimile has preserved the density differences in the original. The features common to both radiographs are substracted from the image as viewed, leaving only those features wherein the radiographs differ, principally the radiopaque detail. The two superimposed radiographs (mask and the second radiograph) can then be used to produce, photographically, a third, subtracted film, which exhibits ideally only the radiopaque detail.

The drawbacks and limitations of this prior art sys- In contrast, the new and novel method of the invention provides for making a subtracted image radiographic record while making only one radiograph, the finished picture, eliminating the making of the base film, the reverse tone facsimile andthe diagnostic film and eliminating the superimposition of the base and diagnostic films.

' In the method of the present invention,- a first electron radiographic exposure is made in the conventional manner to produce an electrostatic image on the dielectric receptor sheet. Then the polarity of the voltage applied to the electrodes is changed and a second exposure is made, with the contrast material having been introduced into the object of interest between the exposures. The second exposure with the reversed polarity produces a second latent electrostatic image on the dielectric receptor which is the exact negative of the preceeding latent image produced by the first exposure, except in those areas where the X-ray flux patterns differ, the differences in X-ray flux patterns being produced by the contrast medium present in the object during this second exposure.

Thus, at the end of the second exposure, the dielectric receptor has zero or substantially zero electrostatic charge everywhere except in those areas in which the two exposures differed. The subtraction of one image from the other has been performed by charge cancellation, with exact registration being automatically obtained by maintaining the imaging chamber and object stationary between the two exposures. Instead of the three separate pictures required in the conventional systems, only the final subtracted picture is produced by developing xerographically the electrostatic image after the second exposure. The subtracted image can be produced in the time required to make two electron radiographic exposures, since the time required for polarity reversal is negligible.

Accordingly, it is an object of the present invention to provide such a new andimproved method for making a subtracted image radiographic record and to provide a new and novel apparatus for making such a record.

The single FIGURE of the drawing illustrates an electron radiographic system with X-ray source, imaging chamber and voltage supplies and incorporating the presently preferred embodiment of the invention.

X-rays are directed from a source 10 past the object 11 being X-rayed to the imaging chamber 12, which may be conventional in design such as that disclosed in the aforementioned copending application.

A typical imaging chamber includes a housing 13 carrying a first electrode 14 on an insulator 15, with a second electrode 16 carried on another insulator 17. The dielectric sheet receptor 18 may be carried on the electrode 16, with gas introduced into the chamber at 19 filling the gap between the electrodes.

A high voltage supply 25 is connected acrossthe electrodes 14, 16 by means of a polarity reversing unit such as a double pole double throw switch 26. The X-ray source 10 may be energized from another high voltage supply 27, with both supplies 25 and 27 turned on by a control switch 28 to provide an exposure.

In operation, the object 11 is placed in position between the source 10 and the imaging chamber 12, the dielectric receptor 18 is loaded into the imaging chamber, and an appropriate gas charge is introduced into the chamber. The switch 26 is actuated to connect a voltage of one polarity from the supply 25 to the electrodes 14, 16. By way of example, the switch 26 may be thrown to the up position connecting the positive output from the supply 25 to the electrode 16 and connecting the negative output to the electrode 14. The system is then ready for the first exposure which may be produced by actuating the control switch 28.

This first exposure produces a first latent electrostatic image on the receptor 18. v

The switch 26 is moved to the opposite position, connecting the positive output to the electrode 14 and connecting the negative output to the electrode 16. The system is then ready for the second exposure. The contrast material may be introduced into the object 11 between the two exposures. The second exposure is produced in the same manner as the first exposure, resulting in a second latent electrostatic image on the receptor 18. The receptor is now removed from the imaging chamber and developed by conventional xerographic techniques to provide a visible image. This visible image will be that portion of the object receiving the contrast medium between the two exposures. Under ideal operating conditions, all other areas of the receptor will have received two charges of the same magnitude and opposite polarity which exactly cancel each other and leave the receptor clear. Thus it is seen that a complete subtracted image record can be made while requiring only a single picture and eliminating all registration problems.

I claim:

1. A process of making a subtracted image radiographic record of an object with an electron radiographic system having an imaging chamber with spaced electrodes and an ionizable X-ray opaque fluid in the gap between the electrodes, a dielectric receptor at one of said electrodes, and a dc power supply, including steps of:

connecting the power supply across the electrodes with a first polarity;

making a first exposure to produce an electrostatic image on said receptor;

connecting the power supply across the electrodes with the opposite polarity;

making a second exposure to produce another electrostatic image on said receptor; and

developing the resultant electrostatic image on the receptor which image is a record of changes taking place in the object between the first and second expost'tres.

2. The process of claim 1 including the step of introducing a contrast material into the object between said first and second exposure steps. 

1. A process of making a subtracted image radiographic record of an object with an electron radiographic system having an imaging chamber with spaced electrodes and an ionizable X-ray opaque fluid in the gap between the electrodes, a dielectric receptor at one of said electrodes, and a dc power supply, including steps of: connecting the power supply across the electrodes with a first polarity; making a first exposure to produce an electrostatic image on said receptor; connecting the power supply across the electrodes with the opposite polarity; making a second exposure to produce another electrostatic image on said receptor; and developing the resultant electrostatic image on the receptor which image is a record of changes taking place in the object between the first and second exposures.
 2. The process of claim 1 including the step of introducing a contrast material into the object between said first and second exposure steps. 